Hydraulic Press and Safety

I have a project where we are going to update some hydraulic presses. Currently there are some old GE 90 Micro and Modicon PLC’s controlling the presses. These will be replaced with GE VersaMax PLC’s.

Current Operation

Currently the operator has to press two buttons to lower the press until it begins pressing the product at which point he can walk away and the press will compress the product to the desired height and then has a dwell of about a minute. After the dwell time is complete the press raises and the operator again presses the product, this is repeated 10 times. After the product is pressed 10 times it is taken out and the cycle is repeated.

New Operation

For the new operation the operator will place the product in the press, hit a button and the press will automatically press the product 10 times. Once this is complete the operator will change out the parts and repeat the process. The press will have a light curtain, touch screen and a PLC.

Question

Obviously you can’t have the PLC control the safety of the Machine, that would defeat the purpose. So the Light Curtain and E-Stop circuits will be completely separate from the PLC control. Here is the thing though, I have been told that in order for a Hydraulic press to be considered safe by OSHA you have to have “redundant” control meaning two PLC’s and dual solenoid valves on all the hydraulics. So if one of the PLC’s fail the machine shuts down.

Has anyone heard of this? Is it true, I haven’t worked on presses before and since we are going to redo this press I prefer to do it right.

The guys telling me this are outside vendors who are hoping to get some work. Whereas I am more interested in learning how it should be and doing it myself. Obviously they don’t want to give me free consulting and I completely understand that but at the same time I don’t want to be paying for stuff that I don’t need.

If anyone knows where I can get some information on what is considered a SAFE setup of a Hydraulic press I would greatly appreciate it.

Thanks.

Hi,

I, too, do not have experience with hydraulic presses, but what you are hearing does smell a wee bit fishy to me as well.

I found this document which details OSHA's requirements for third-party testing and evaluation of the safety of hydraulic presses. Very interesting reading. zzzzz

Just thought it might shed some light. Looks like you are in for an education anyway.

Keep us posted. I for one would like to hear what you come up with.

Steve

Hi glaverty,

1. Can I assume now that the press only has relays for the logic?

2. Will the vendor quote the OSHA code?

3. My thought, so long as the manufactures safety sequence is maintained, I don't see any issue on your approach, so long as any intrusion / fault will have the Press goto a "safe state"

(Light Curtain and E-Stop circuits completely separate from the PLC control is a good approach, but have the PLC monitor state/status of them though)

Added
OSHA SEARCH ON HYD PRESSES

I think that you only need the redundant plc's if an action taken in a safety situation. If all you want to do is stop it when the light curtain broken, wiring those ouputs through the LC circuit should be sufficient.

Thanks for the links guys that helps out a little.

Rick,

That is exactly what I thought but I was getting the impression that that wasn't the case.

I met with the guy that does the PLC programming for this company and he was pretty straight forward about stuff. He said that really OSHA doesn't require this stuff but it looks like they are heading that way so they try to stay ahead of the game.

I asked him how he goes about programming the redundancy. Bascially he has two PLC's communicating with each other. One PLC has all I/O on it and the other is really just a CPU. So when it comes time to fire a solenoid or what not he has a bit fire in PLC A it communicates to PLC B and then PLC B communicates back to PLC A that it recieved the bit.

I see it as looking something like this.

Ladder in PLC A

Activate

Solenoid                        PLC A OK

-| |------------------------------( )-

                   | 

                   |

                   |  PLC B OK  Solenoid

                   |----| |--------( )-

Ladder in PLC B

PLC A OK                        PLC B OK

-| |-------------------------------( )-

'

He said that really you only need the redundancy for times when you fire something that is a safety concern. Other things that you might have going on that aren't a safety concern such as indicator lights, horns, etc. you wouldn't need to do the communication check.

This doesn't seem like redundancy to me, I would have thought that you would have two PLC's running the exact same code and if they didn't agree the machine wouldn't operate. This seems more like a communication check than anything.

I guess you could program the second one to check what the first one is doing and make it a little more robust than just checking that there is communication between the two. Check that there is communication and that they are in agreement on the state of the inputs which correspond to the firing of the outputs. That would make sense to me.

If anyone has programmed anything like this I would love to hear what your take on it is or anyones opinions and thoughts.

hello,ive done a bit of work on hydraulic folding machines,sheet metal
etc.its probably different the safety rules in europe,but since a few years back all the hydraulic valves have centre posicion sensors on each end of the valve.thus before reinitiating another cycle the valves have to be in there top dead centre posicion,all of these
inductive sensors went through a safety module which later with the tie down module gave the order to repeat the cycle.they didnt use plcs,all relay logic,not a lot.these were 4 and 5 axis machines,
with a dedicated cnc to do all the posicioning.regards tony

You may want to take a look at this site: http://www.sti.com/safety/index.htm
I dont know alot about their products but the light curtains and plc are the plc they use to sell are the same brand AB markets now.

I have recently taken a few courses/seminars on safety, I still dont know anything but redundancy is used on the safety side of it. I havent seen a rule that requires the use of dual plc's but it is common practice to use redundant contactors/relays in the hardwiring and a safety monitoring relay or safety plc is used to determine if their is a fault and not allow a restart.

Thanks Ron,

That was one of the first places I went when I started this project, they had some good stuff and I thought I was doing it right, then these guys start talking redundant plc's and dual solenoids which pertains more to the Hydraulic press itself than the light curtains and such.

glaverty,

You can contact KTI Technologies to discuss your safety requirements for your press. Their web site is http://www.ktitechnologies.com/

They offer all sorts of safety equipment from light curtains to a redundant PLC.

Hope this will help you.

God Bless,

Stephen Luft

As the song goes:
"ya gotta know when to hold em, and when to fold em"

I let the Vendors do the Job. The company will get what they pay
for without hassels, and you get company paid education.


'WIN-WIN'

Hey Glaverty,

I don't have a ton of experience on hydraulic presses, but played with a few in my time...

The person that told you the redundancy is 'likely' a future OSHA regulation is correct. From what I understand, OSHA has been pushing for this for a few years, but there are grandfather clauses, etc.. that need to be worked out. Given the fact that most large presses run for quite a long time, it's hard to implement this process without a lot of screaming from manufacturers....

If it helps, most of my experience with large presses is in injection molding. Some of these presses will go to 600 tons, and higher, but they usually only have one bank of hydraulic valves for their process. What they usually do is allow you to activate them manually in case of an emergency, or go to a certain state when power is off or interrupted. The newer presses I have worked with do come with a battery backup, with a cpu backup like you described. They don't have 2 banks of plc's, but there are a ton of checks that come with them. On injection molding, they use a physical door that will interrupt all power when opened. This seems to be perfectly acceptable with OSHA.

IMHO, I would setup the presses to where when power is interrupted, they will 'cycle up' or to an open position. I would add the second cpu for the check, and do as you said with the light curtains. Make them an external circuit from the PLC. HTH.

Hoot

European highest category.
-Operation needs people presend and probability to be hurt is evident-.

Light curtain works as doubled emergency stop.
Ems-realys double-disconnect all solenoid-valves and hydraulic pump starter.
Pump starter have controlled with 2 serial main contactor.
EMS-relay reset is possible only when all contactors have released.

Normal PLC-solution can be used with 2-hand start-control.

This is recommendation only.
For safety reason the pump is for this machine only, -not common-.
Hydraulic oil is liquid, so generally it can't produce pressure without the pump -if there are not pressure-accumalator(s)-.

In addition to the safety items hoot mentioned, horizontal hydraulic presses (injection molding machines) also have a mechanical device to prevent the press from closing with the front gate closed. I'm not sure of it's "correct" name. We always called it a "jam bar". Simply a round bar with notches similar to this:

                    +---+

                    |   |

                    | A |

                    | R |

                    | M |

                    |   |

                    +---+



      +--+       +--+       +--+

     /   |      /   |      /   |

____/    |_____/    |_____/    |______

         |          |          |     /

         |          |          |    /

         |          |          |   /

         |          |          |  +---+

         |          |          |     /

         |          |          |    /

____     |_____     |_____     |___/

    \    |     \    |     \    |

     \   |      \   |      \   |

      +--+       +--+       +--+

.
Opening the gate causes the arm to drop down to "jam" the bar, preventing the press from closing:

                    +---+

                    |   |

  CLOSE --->        | A |

                    | R |

      +--+       +--| M |   +--+

     /   |      /   |   |  /   |

____/    |_____/    |___|_/    |______

         |          |          |     /

         |          |          |    /

         |          |          |   /

         |          |          |  +---+

         |          |          |     /

         |          |          |    /

____     |_____     |_____     |___/

    \    |     \    |     \    |

     \   |      \   |      \   |

      +--+       +--+       +--+

.
How's THAT for ascii art?...

This is just an additional method of preventing the press from closing if all else fails. Perhaps you could implement something similar on your vertical press? Sorry, I ain't gonna draw it... Just look at the above drawing and tilt you head to the left....

For some reason, I'm picturing the safety brake mechanism on the original Otis elevator... :nodi:

beerchug

-Eric

glaverty,

You can use a PLC to provide safety on a machine, provided that you use the proper hardware. There is safety PLC manufactures such as Piltz and there are I/O modules, which have safety relays, installed in them such as the Omron CQM1-SF200 or the CS1W-SF200 safety relay PLC modules.

However when dealing with safety there are many things which needs to be considered, such as are you or is someone at your company qualified to do a safety risk assessment? This must be done and keep on file, you must show all of your data as to how you decided to mount your light curtain or safety devices where you did. You have to have all of your calculations as to the time it takes the ram to stop once the circuit has been de-energized, and your calculations as to how far away your protective devices should be mounted.

Since you will be accepting the responsibility for the safety of this machine you must also sign this documentation as to its accuracy. This will also allow anyone who is injured to name you in his or her lawsuit.

Because of all of what I have mentioned and several other factors I have decided to use a safety engineering company who specializes in press packages for all of my press projects. I am sure that there are several of these companies out there how ever I use Rockford Systems out of Rockford IL. If you are interested I will be glad to provide you with the companies phone number and a contact name for you to discuss your application with they are very knowledgeable and helpful.

Thanks,

Mike

Eric,

I'm impressed, great art! And I can't believe I forgot the drop/jam arms...

Hoot